This invention relates to an X-ray generating apparatus equipped with a power source circuit comprised of a bridge type inverter circuit having a resonance circuit.
It is known that an X-ray generating apparatus includes a power source circuit comprised of a bridge type inverter circuit having a resonance circuit in its switching section. One form of the X-ray generating apparatus is disclosed in U.S. Pat. No. 4,225,788.
This type of X-ray generating apparatus is adapted to obtain a DC voltage by rectifying and smoothing an AC input signal supplied from a power source (in general, a commercial power source). The DC voltage is intermittently supplied to a resonance circuit, including a primary winding of a transformer and resonance capacitor, by an alternate switching operation of a pair of series-connected switching elements, such as thyristors. As a result, an AC output is induced in the secondary winding of the transformer and, after being rectified, supplied to the X-ray tube. A rectifying element (diode) is normally connected in parallel with the switching element in a cathode-to-anode (i.e., an inverse parallel) fashion.
In the X-ray generating apparatus, if two series-connected switching elements overlappingly conduct for some reason or other, an excess short-circuit current flows through the switching elements, thus leading to a failure of the switching elements.
In order to solve this problem, the first and second switching elements are so controlled that after the first switching element is completely turned OFF, the second switching element is turned ON.
Even in this method, the problem may still arise, for example, due to a variation in the characteristic of the circuit elements and due to an instability of a gate pulse which is supplied from the control means for the switching elements.
In order to protect the switching element from an excess current, use may be made of a fast-break fuse or a circuit breaker. The fast-break fuse capable of an adequately high speed operation is normally costly. Furthermore, the circuit breaker is slow to respond and, therefore, it is not possible to provide adequate protection.
A switching element, such as a thyristor, takes a predetermined time from the ceasing of the ON current due to the turning OFF of the thyristor until it fails to conduct even if a forward voltage has again been applied. This predetermined time is referred to as a turn-off time (reverse recovery time) and is of the order of tens of .mu.s even for an ordinary high-speed type.
In consequence, the closing timing of the next switching element is restricted and, for this reason, the ripple factor of the X-ray tube voltage waveform is increased, causing a fall in an output dose.